a few months ago my friend told me that the expression of "do you understand?" is impolite expression using in classroom. what do you think?. actually i often used this expression to check the students understanding about the material that i've given. if it is impolite what expression should i use?
thanks

It's not so much that the question is impolite, more that it is not very helpful.

When you say: "do you understand?", students have a tendency to say "yes" whether they understand or not. The ones who do understand will say "yes" very quickly and then the ones who don't understand may not want to speak out.

It would be more helpful to find another way of checking understanding. eg by asking students to explain it back to you or by setting a task that sees if they can use the language correctly.

To be honest I've used this expression many times myself, I wouldn't call it "impolite". However I think it shouldn't be addressed to the whole class but a single student during a conversation for example.